(1) Field of the Invention
The present invention relates to transparent thermographic recording films, and more specifically, it relates to a topcoat for transparent thermographic recording films which are to be imaged with a thermal printhead and which exhibit good anti-stick properties, are scratch resistant, water resistant, substantially craze-free and low in haze.
(2) Description of the Related Art
Color-forming di- and triarylmethane compounds possessing certain S-containing ring closing moieties, namely a thiolactone, dithiolatone or thioether ring closing moiety are disclosed in European Patent No. 250,558 and U.S. Pat. No. 4,904,572. These dye precursors undergo coloration by contacting with a Lewis acid material, preferably a metal ion of a heavy duty, particularly silver, capable of opening the S-containing ring moiety to form a colored metal complex.
As disclosed in the above-cited European Patent, the ability of these dye precursors to form a colored dye almost instantaneously when contacted with Ag+ renders them eminently suitable for use as color formers in thermal imaging systems employing organic silver salts, such as silver behenate. In these systems, color formation is particularly efficient since it is effected by a phase change, i.e., effected by the melting of the organic silver salt to provide the Ag+ necessary for coloration rather than requiring a change of state.
As disclosed in European Patent No. 250,558, mentioned above, these thermographic recording films preferably include a heat-fusible organic acid material. U.S. Pat. No. 4,904,572 discloses 3,5-dihydroxybenzoic acid as a preferred heat-fusible organic acid.
The above described thermal color-forming system requires a thermoplastic binder, e.g. polyvinylbutyral, in order for the image-forming chemistry to function in a thermal printing environment. When imagewise heating is accomplished by means of a thermal printhead, the thermoplastic binder is in direct contact with the thermal printhead during imaging. Since thermoplastic binders soften upon the application of heat, they tend to stick to the thermal printhead during imaging.
This "sticking" interferes with the printing, adversely effects image quality, and can cause damage to the printhead.
A number of ways to prevent sticking between a binder and a thermal printhead during printing have been suggested for various thermographic recording films. Many of these employ a protective or anti-stick topcoat comprising silica over the thermographic color-forming layer. These topcoats contact the thermal printhead during imaging to prevent "sticking". Another way to prevent sticking has been to employ a surface active agent to add anti-stick properties. However, these silica containing topcoats and surface-active agents have drawbacks and/or do not perform adequately when the binder employed in the coloring system is polyvinylbutyral and the support used for the thermosensitive recording film is a transparent support.
For example, low surface energy materials such as silicone polymers exhibit good anti-stick properties. However, the useful silicone polymers are relatively low molecular weight silicone polymers which have a tendency to be migratory and thus cause problems, e.g., they transfer to the back of the film if it is rolled for storage or to the back of the adjacent film if stored in sheets. In addition, because these silicones are polymers, their properties change with changes in moisture and temperature and therefore, their performance is not consistent under all conditions.
U.S. Pat. No. 4,583,103 issued Apr. 15, 1986 and U.S. Pat. No. 4,820,682 issued Apr. 11, 1989 disclose protective topcoats for heat-sensitive recording papers containing a binder comprising silicon modified polyvinylalcohol and colloidal silica and/or amorphorous silica. The above patents also disclose topcoats wherein said colloidal silica contains silica grains having an average particle size of from about 10 millimicrons (m.mu.) to 100 m.mu. (1 m.mu.=1 nanometer (nm)) and the amorphous silica has primary grain size of about 10 micrometers (.mu.m) to 30 .mu.m (1 .mu.m-10.sup.3 nm). These topcoats are disclosed as providing good printing densities, resistance to various chemicals, oils and water, and anti-sticking and anti-blocking properties. In addition, the latter patent discloses the topcoat as exhibiting excellent transparency and describes it for use on a transparent base. However, the lowest level of haze reported is 16%, a level which is higher than desirable for overhead transparency (OHT) applications.
Published UK Patent Application No. 2,210,702 having a publication data of June 14, 1989 and assigned to the same assignee as the latter two patents, discloses a heat-sensitive recording material which, when it employs a topcoat as described above, e.g., silicon modified polyvinylalcohol and colloidal silica, reports a level of haze as low as 8%.
However, when polyvinylbutyral is used as the binder for the color-forming materials of this invention, and a topcoat as described above, i.e. silicon modified polyvinylalcohol and colloidal silica, is employed to prevent sticking, there is poor adhesion between the topcoat and underlying polyvinylbutyral layer, as well as poor scratch resistance of the resulting film. In addition, the silicon modified polyvinyl alcohol binder is water soluble and can be rubbed off with water.
U.S. Pat. No. 4,985,394 issued Jan. 15, 1991 discloses a topcoat for a thermosensitive recording material which comprises at least one inorganic pigment selected from the group consisting of silica and calcium carbonate, each having an average particle diameter of 0.1 .mu.m or less, and a water-soluble binder, formed on the thermosensitive coloring layer. Many of these topcoats have problems of inadequate transparency and/or adhesion when coated over the polyvinylbutyral color-forming layer of the present invention.
Thus, a hard, durable topcoat is required which can be placed over the polyvinylbutyral color-forming layer(s) to prevent sticking of the polyvinylbutyral to the thermal printhead during printing, and which is resistant to scratching and crazing and also exhibits high transparency.